Typically, advanced high performance materials must remain stable and retain structural integrity over a wide range of environmental and temperature conditions. Many such advanced performance materials contain, or are attached to, a surface substrate containing open pores and channels, which act as a thermal or corrosion barrier with respect to underlying materials. Examples of use of such high performance materials include components incorporated into turbine engines such as used in aircraft, aerospace, and energy and power generation applications. In such a turbine, hot combustion gasses contact components such as rotors, turbine blades, vessels, and shrouds. Other applications of high performance materials include ceramics, ceramic matrix composites (CMC), plasma-sprayed ceramic coatings, and refractories.
Materials and articles containing inorganic porous surfaces are used extensively in many industrial applications to provide thermal insulation, heat resistance, reduced weight, or increased toughness. Such a porous surface material may be metallic or ceramic. An example of such an article is an external coating of a porous material on an underlying metallic or ceramic material. Typical porous industrial articles contain open pore channels of varying sizes (dimensions of several micrometers down to nanometers in pore diameter) and geometries. These open pore channels often compromise the chemical, mechanical, and electrical properties of these articles. An aspect of this invention is an internal coating within a pore structure that can provide environmental protection and can maintain or improve the chemical, mechanical, and electrical properties of such articles
Specific applications of high performance materials or substrates useful in this invention include, but are not limited to, thermal barrier coatings, thermal protection systems, radiation shielding systems, heat rejection systems, lightweight thermally stable structural members and systems for active or passive functionality, environmental barrier coatings and systems, anodized metals, metallurgical overlay coatings, plasma spray coatings, ceramic metal composite coatings, electron beam physical vapor deposition derived coatings, and slurry coatings. Articles embodying these applications may include, but not limited to, refractory ceramics and composites, turbine engine components, exhaust or airframe components, and other aerospace or utility land-based power generation turbine hardware.
A purpose and function of these high performance materials is based in the high stability of the materials to harsh environments such as to extremely high temperatures that may be above 500° C., typically above 800° C., and especially above 1000° C. Typical harsh environments include an oxidizing, reducing, high temperature, or vacuum atmosphere, and additional atmospheric components such as water (vapor or liquid), and common atmospheric contaminants such as dust, dirt, sand, ash, fuel additives/contaminants or biofuel derivatives, and various organic compounds. Further, porous ceramic materials or coatings used at elevated temperatures for extended periods may experience sintering in which pores coalesce. Such sintering may result in variation of thermal insulation properties and additional stress that may degrade the integrity of the material. Added requirements imposed by such harsh environmental conditions associated with the high temperatures are directed to stability of the materials with respect to conditions, such as oxidation, corrosion, embrittlement, fatigue, mechanical wear, stress cracking, structural changes such as sintering or densification, loss of adhesion or loss of material (mass or thickness), reduction, and chemical reaction.
Pore coalescence is the accumulation of multiple pores to a common proximity and subsequent union of the pores to form a lesser number of larger pores. This process is favored when the mobility of pores within a material is sufficiently high to allow the pores to accumulate in proximity to one another; the joining step then will occur quickly once the proximity of neighboring pores is sufficient. This process can be substantially impeded through increased bonding within the bulk of a material, which can be accomplished through heat-treating or reaction-based doping processes.
A grain is an amount of material that exhibits a consistent phase and crystal structure orientation over some finite scale, typically on the order of 10 nanometers to 100 μm, and often referred to as a crystallite. Polycrystalline materials are comprised of a multitude of small grains. Grains comprise all ordered materials, including ceramics, metals, and many polymers.
Grain growth is the process of the joining of multiple small grains, which are in proximity to one another to form a lower number of larger grains. This process is favored when grain boundary mobility is sufficiently high to allow grain boundaries to impinge into neighboring grains. Grain growth processes decrease porosity and impart mechanical stresses into the bulk of the material, which often have detrimental consequences, often resulting in the destruction of intentionally engineered material architectures and stress fields.
Other porous materials that experience corrosion and sintering at high temperatures include refractory materials, such as used in steel manufacture or used in furnaces, kilns, reactors, or incinerators. A typical refractory is a metal oxide such as oxides of aluminum, magnesium, calcium, titanium, zirconium, and chromium. Other refractory materials include silica, silicon carbide, and graphite. A refractory material may contain mixtures of refractories.
A thermal barrier coating may be prepared from a ceramic material, such as a chemically (metal oxide) stabilized zirconia. Examples of such chemically stabilized zirconia include yttria-stabilized zirconia, scandia-stabilized zirconia, calcia-stabilized zirconia, magnesia-stabilized zirconia, and combinations thereof. The thermal barrier coating of choice, typically, is an yttria-stabilized zirconia ceramic coating. A representative yttria-stabilized zirconia (YSZ) thermal barrier coating usually contains about 7 wt. % yttria and about 93 wt. % zirconia. The thickness of the thermal barrier coating depends upon the metal part or component it is deposited on, but is usually in the range of from about 0.5 to about 2 millimeters (typically 0.1 to 1 mm) thick for high temperature gas turbine engine parts.
To prevent turbine components from getting too hot, thermal barrier coatings (TBC's) often are coated onto various surfaces of the turbine components to insulate the components from the high temperatures in the hot gas path. TBC's are an increasingly important component in current and future gas turbine engine designs because of the higher operating temperatures in gas turbine engines. Examples of turbine engine parts and components for which such thermal barrier coatings are desirable include turbine blades and vanes, turbine shrouds, buckets, nozzles, combustion liners and deflectors, and the like. These thermal barrier coatings typically are deposited onto a metal substrate (or more typically onto a bond coat layer on the metal substrate for better adherence) from which the part or component is formed. The TBC reduces heat flow and limits the operating temperature experienced by such metal parts and components. A suitable metal substrate typically is a metal alloy such as a nickel-, cobalt-, and/or iron-based alloy (e.g., a high temperature super alloy).
Although significant advances have been made in improving durability of thermal barrier coatings on metal substrates such as for turbine engine components, these coatings still are susceptible to various types of damage, including objects ingested by an engine, erosion, oxidation, and attack from environmental contaminants. In addition, in trying to achieve reduced thermal conductivity, other properties of the thermal barrier coating can be adversely impacted. For example, the composition and crystalline microstructure of a thermal barrier coating, such as those prepared from yttria-stabilized zirconia, can be modified to impart to the coating an improved reduction in thermal conductivity, especially as the coating ages over time. However, such modifications also can unintentionally interfere with desired spallation resistance, especially at the higher temperatures that most turbine components experience. As a result, the thermal barrier coating can become more susceptible to damage due to the impact of, for example, objects and debris ingested by the engine and passing through a turbine sections thereof. Such impact damage eventually can cause spallation and loss of the thermal barrier coating.
Chemical-based pore obliteration can occur when a porous material is involved in chemical reactions with an externally introduced reactant, which often is a contaminant species. Because environmental conditions and contaminants are present in widely variable ranges, a great number of possible reactions are common in advanced material applications and are magnified with increasing temperatures. A common reactant for thermal barrier coatings, for example, is a calcium aluminosilicate-based glassy material, which is found as a constituent in sand and dust. The starting sand and dust material is found commonly in the atmosphere in areas around the world, and may be ingested into turbine engines on aircraft as well as stationary power generating equipment. Sand or debris is melted in the high temperature environment inside a turbine engine, and the resulting molten material is deposited onto hot surfaces of internal structures of a turbine engine, which are coated with porous thermal barrier coatings. Molten sand debris, the predominant constituent of which is a slag-like and glassy mix of calcium-magnesium-aluminosilicates (referred to as “CMAS”), infiltrates into porous structures of a thermal barrier coating upon contact. Ceramic oxide-based thermal barrier coatings typically are very reactive with CMAS compositions and, essentially, dissolve into solution in the CMAS. Reaction products are precipitated from the dissolution melt that are more stable than the original CMAS+TBC mixed system. Thus, the originally designed porous architecture of the TBC is eliminated as the chemical reaction with CMAS progresses and functionality of the starting TBC material declines.
Solidified CMAS causes stresses to build within the thermal barrier coating, leading to partial or complete delamination and spelling of the coating material and, thus partial or complete loss of the thermal protection provided to the underlying metal substrate of the part or component.
Pores, channels, or other cavities that are infiltrated by such molten environmental contaminants can be created by environmental damage or even the normal wear and tear that results during the operation of the engine. However, the porous structure of pores, channels, or other cavities in the thermal barrier coating surface more typically is the result of processes by which the thermal barrier coating is deposited onto the underlying bond coat layer-metal substrate. For example, thermal barrier coatings that are deposited by air plasma spray techniques tend to create a sponge-like porous structure of open pores in at least the surface of the coating. By contrast, thermal barrier coatings that are deposited by physical (e.g., chemical) vapor deposition techniques tend to create a porous structure comprising a series of columnar grooves, crevices or channels in at least the surface of the coating. This porous structure can be important in the ability of these thermal barrier coatings to tolerate strains occurring during thermal cycling and to reduce stresses due to the differences between the coefficient of thermal expansion (CTE) of the coating and the CTE of the underlying bond coat layer/substrate. CMAS mitigation coatings often are needed for gas turbine operation above 1200° C. Many turbine engines operate in this temperature regime. Without CMAS mitigation, functionality of a TBC often is compromised, and the component may fail. Coatings of alumina have been used to mitigate CMAS attack; however, this approach has not been successful largely due to poor distribution of alumina throughout the porous network. Addition of alumina to the CMAS increases its melting point, thereby arresting its molten flow and, consequently, its degrading effect. Thus, in this case, the alumina acts as a sacrificial layer to arrest further flow or degradation, which effectively protects the substrate from CMAS attack.
In general, most porous ceramic coatings and materials are designed to retain porosity to exploit mechanical properties (toughness), low density (e.g., in aerospace/transportation), filtration, and thermal insulation; however, these porous coatings are prone to environmental degradation. There is a need selectively to deposit or plug the porosity with an inert material without substantially altering the overall porosity level or microstructure to retain the properties. Also, there is a need to protect pore walls from reacting with environmental species. Such protection needs to be done preferably with a high temperature stable material that is inert, chemically compatible with various substrate materials, and remains morphologically stable upon long-term high temperature exposures.
Porous ceramic materials and composites also may exhibit lower mechanical strengths compared to their higher density counterparts. Such lower strengths are partially attributable to micro-scale flaws (on pore walls or interiors) contained within porous microstructures. The internal coatings of the present invention may heal such flaws and provide an improvement in mechanical strength and toughness of the porous substrate material. External porous coatings, such as thermal barrier or insulation coatings on a structure also are subject to severe erosion in harsh environments. Internal pore coatings of this invention may improve performance of these external coatings under erosion conditions.
Anodized aluminum is commonly used in many consumer and industrial products to impart corrosion protection. However, anodized aluminum has a thin amorphous alumina surface that contains micro-scale open porosity that serves as pathways for moisture, salt, or other environmental contaminants to penetrate and attack the underlying non-anodized metal. There is a need to seal at least partially the open porosity in these coatings to enhance their corrosion protection.
Porous materials and coatings also are used as dielectric materials or to provide electrical insulation in many use applications. Presence of open pore channels often may compromise the durability of the materials over time due to ingress of environmental species that alter the electrical properties either by the presence of such species or through reaction with pore walls. There is a need for a system that creates long term stability during operation and service of porous materials to maintain their designed electrical properties.
There is a need for an effective system to protect porous advanced high performance materials against harsh environmental conditions experienced during use without significantly affecting physical properties of such materials such as strength, weight, thermal conductivity, wear tolerance, stress cracking resistance, and the like. Further, there is a need to minimize sintering in porous materials at elevated temperatures.
Conventional attempts to mitigate harsh environmental conditions on porous advanced high performance materials include placing a coating or a metallurgical overlay coating on the exterior surface to completely fill pores of such materials. However, such exterior coatings (e.g., a topcoat) tend not to be hermetic or tend to crack or spall during thermal cycling such that the sealing is not effective or durable. Other known protective techniques, such as use of nanoparticle-based alumina slurries to infiltrate fine pores of a thermal barrier coating, do not yield conformal coatings on the pore walls and do not produce sufficient environmental protection. Use of metal organic chemical vapor deposition (MOCVD) alumina coatings has been described to infiltrate the finer pores in thermal barrier coatings. However, these coatings have high cost and may not be suitable for large complex-shaped components, and such coatings may not have an ability to infiltrate fine pores without adversely affecting TBC properties such as strain tolerance. Further, this technique may not be suitable for use in the field or for repair. Slurry-based systems to deposit alumina-based films into a porous substrate may not be suitable to deposit conformal films on pore walls or may not penetrate very small pores or uniformly infiltrate through the depth of the porous substrate and, thus, may not provide adequate environmental protection. Presently, there are no practical methods of depositing clear, low viscosity, high yield solutions to form good quality alumina-based films onto pore walls of a porous substrate.
This invention provides a solution to providing an internal coating of a porous substrate to form a superior environmental protection system, that further is capable of maintaining or improving chemical, mechanical, and electrical properties without significantly affecting desirable properties of a porous advanced material. A pore wall barrier internal coating and selective blocking of mesopores may be created without having a detrimental effect on the design properties of such materials.